Self-coloring anodic oxidizing process for aluminum and for alloys thereof

ABSTRACT

An improved self-coloring anodic oxidation process for aluminum and its alloys utilizing an electrolytic bath in which the electrolytic cell is subdivided by a porous baffle into an anodic and a cathodic compartment, the improvement comprising maintaining the electrolytic solution in the cathodic compartment at a temperature lower than that of the anodic compartment.

United States Patent Micheletti Nov. 27, 1973 [54] SELF-COLORING ANODICOXIDIZING 3,616,311 10/1971 Barkman et al. 204/58 PROCESS FOR ALUMINUMAND FOR ALLOYS THEREOF OTHER PUBLICATIONS 75 Inventor; Ciro Mi h l tfiNovara, Italy The Surface Treatment and Finishing of Aluminum by Wernicket al. 1964, page 314. al., [7 3] Assignee: Societa Per LEsercizioDelllstltuto Sperimentale Metalli Leggeri, Milan, Italy PrzmaryExammer1ohn I-I. Mack Assistant Examiner-R. L. Andrews [22] Fled: 14,1972 Attorney-Richard K. Stevens et a1.

[21] Appl. No.: 234,683

[57] ABSTRACT [30] Foreign Application Priority Data I M a r 5 197] ha]21754 N71 An improved self-colorlng anodic oxidation process y foraluminum and its alloys utilizing an electrolytic [52] U S 204/58 bathin which the electrolytic cell is subdivided by a [51] 1". .Cl 9/02porous baffle into an anodic and a cathodic compart [58] Field 262 274ment, the improvement comprising maintaining the electrolytic solutionin the cathodic compartment at a [56] References Cited temperature lowerthan that of the anodic compart- 1 Claim, No Drawings SELF-COLORINGANODIC OXHDIZING PROCESS FOR ALUMINUM AND FOR ALLOYS THEREOF The presentinvention relates to an improvement in self-coloring anodic oxidizingprocess for aluminum and its alloys, an improvement that reduces theconsumption of the components of the electrolytic system used andsubstantially eliminates precipitate formation from said system.

It is known that aluminum and its alloys may be coated with layers ofoxide of considerable thickness and high resistance, by an electrolytictreatment in aqueous solutions in which the aluminum acts as an anode.

Moreover, it is also known that by using as electrolytes solutions basedon carboxylic acids of the aliphatic series, it is possible to obtainself-colored layers by transformation and incorporation in said layersof the constituents and alloying elements present in the material to beoxidized.

The self-coloring anodic oxidizing processes so far suggested, amongstwhich may be particularly mentioned, are the MS-process, based on abinary mixture of maleic acid and sulfuric acid, the MOS-process basedon a ternary mixture formed of maleic acid, oxalic acid and sulfuricacid, as described in Italian Patent No. 793,929, and in Belgian PatentNo. 660,867, and the TOS-process based on a ternary mixture of tartaricacid, oxalic acid and sulfuric acid, as described in Italian Patent No.720,028, have all in common a serious drawback, that is, to give riseduring the course of the oxidation, to cathodic and/or anodicdecomposition by-products of the aliphatic carboxylic acids used, withthe consequence of heavy consumptions of the components therebyaffected, of an increase in processing costs, and appreciable variationsin the reproduceability of the coloration.

Said decomposition reactions lead in some instances to the completefission of the carboxylic acid or to its transformation to solubleproducts (such as is the case with oxalic acid in the TOS andMOS-processes). In other cases, on the contrary, there takes place aconversion of the affected carboxylic acid into difficulty solubleproducts (such as is the case with maleic acid in the MOS- andMS-processes), with the consequent formation of precipitates in variousparts of the installation and consequent clogging (particularly of theperforated pipes used for the compressed air stirring of theelectrolyte) and a lowering of the exchange capacity of therefrigerating apparatus and of the resin columns used for eliminatingthe aluminum from the baths.

In order to reduce the drawbacks indicated above, it has already beensuggested by the applicant, in Italian Patent No. 846,035, animprovement consisting of placing in the electrolytic cell, used forcarrying out the anodic oxidation process, a porous baffle immersed intothe electrolytic solution used in the process, said porous bafflesubdividing the electrolytic cell into two compartments, an anodic oneand a cathodic one. By this expedient there is achieved a reduction of40 percent in the consumption of the maleic acid in an electrolytis bathbased on maleic and sulfuric acid.

By using the Qbovmmontionecl porous battle, there still remains, even itreduced, the formation of precipitates in the cathodic compartment.

An object of this invention is that of providing an intprovsmont in aself-coloring anodic oxidizing process for aluminum and its alloys, bywhich improvement the above-mentioned drawbacks are reduced.

More particularly, an object of this invention is that of providing aprocess that will reduce to a minimum the formation of precipitates fromthe electrolytic solution and consequently reduce harmful cloggingphenomena in the installation and the reduction of the exchange capacityof the refrigerating apparatus and of the ion-exchanging resin columns.

Still another object of this invention is that of reducing theconsumption of the components of the electrolytic system used, alsoobtaining thereby an improved uniformity of the composition of theelectrolytic system and, thus, a better reproduceability of thecolorings obtained in the anodic coating, without having to resort tofrequent and expensive additions to the electrolyte.

These and still other objects of the invention willbecome still moreapparent to those skilled in the art from the following detaileddescription, and are conveniently achieved through the present inventionwherein the electrolytic cell is subdivided by a porous bafi'le into twocompartments, an anodic one and a cathodic one, and wherein according tothe improvement of this invention the electrolytic solution of thecathodic compartment is maintained at a temperature that is lower thanthat of the anodic compartment.

During the tests carried out in order to examine the cathodic and anodicreaction mechanisms of the diand tri-acid baths affected by thedecomposition of the components thereof, it was, in fact, surprisinglyfound that if the temperature of the catholyte is maintained below theworking temperature of the anolyte, there occurs a heavy decrease in thedecomposition kinetics of the electrolyte components subject tosuch aphenomenon, and, thus, there occurs a substantial drop in theconsumption of the bath components affected by these decompositionreactions.

During said tests there was also observed a considerable reduction ofprecipitates in the cathodic compartment.

The electrolytic solution used in the cell includes a carboxylic acidwhich may be selected from among the following acids: maleic, oxalic,tartaric acid, either along or in a binary or ternary combination, andgenerally in the presence of sulfuric acid.

The porous baffle that subdivides the cell into two compartments, acathodic one and an anodic one, as already described in Italian PatentNo. 846,035 by the same applicant, is of a material resistant to thecorrosive action of the electrolytic system used, preferably beingcomposed of polyethylene, glass or a ceramic material. Said baffle haspores having a diameter comprised between 0.1 and 50;.t.

The difference of temperature or thermal gradient between theelectrolytic solutions of the two compartments, the anodic one and thecathodic one, is in general achieved by cooling down the cathodiccompartment with respect to the anodic one.

Said cooling down may be achieved by circulating a liquid coolant insidea sleeeve surrounding the cathodic compartment, or in a coil immersed inthe cathodic compartment, or by means of any other equivalent suitablemeans.

In case a coil immersed in the cathodic compartment is used, said coil,like the porous battle, will have to be made oi. a material resistingthe corrosive action of the electrolytic system used. Preferably, itis'rnade of the same material of which the cathode is made, e.g., ofstainless steel, graphite or aluminum.

Moreover, the refrigerating system and the cathode may form one singleelement of the cell, the cooling coil being incorporated into thecathode itself.

It has been found that, to obtain the best results, the temperature ofthe electrolytic solution in the cathodic compartment must be kept below22 C.; such a temperature must be, however, higher than the temperaturecausing freezing of the electrolytic solution used in the cell.

During tests carried out, it was found that the use of the porousbaffles combined with the cooling of the cathodic cell, especially inthe case of the MS and MOS processes, offered considerable advantageswithout, however, involving on the other hand too heavy building andmaintenance costs. The advantages of the present invention may be thusconcisely summarized:

l. Elimination of precipitates, in the case of the MS and MOS processes,in the anodic zone, and all the drawbacks derived from suchprecipitates;

2. Considerable reduction, still in the case of the MS and MOSprocesses, of the precipitates also in the cathodic zone;

3. A reduction of consumption (in all mentioned processes) of thecomponents affected by the secondary reactions of the electrolytes;

4. A reduction in ancillary equipment and in their maintenance inconnection with the precipitates.

For an even clearer illustration of the present invention and theadvantages therefrom derived, the following example of an embodiment ofthe invention will now be given.

EXAMPLE The anodic oxidation was carried out at 1.5 A/dm, at 20 C. andfor 45 minutes, in a self-coloring bath of the MS type consisting of anaqueous solution and maleic and sulfuric acid of the followingcomposition:

maleic acid: 300 g/l sulfuric acid: 3 g/l The cell was subdivided by aporous polyethylene bafile having pore-diameters comprised between 7 and1011.. In the cathodic compartment was arranged 6 cathodes consisting ofAISI 316 stainless steel pipes of such a surface as to ensure that thecathodic current density be of at least 1.5 A/dm.

Through these pipes forming the cathodes was made to circulate arefrigerating liquid so as to maintain the temperature of theelectrolytic solution of the cathodic compartment constantly at 17 C.

In the self-coloring bath were oxidized various samples until attainingan aging of the bath of about 70 Ah/l.

After this aging, it was observed that in the anodic compartment noformation of precipitates had taken place and that in the cathodiccompartment the formation of precipitates was considerably reduced.

Moreover, it was also found that the consumption of maleic acid waslower by about 60 percent in comparison with that found in a similarbath without the porous baffle and the cooling of the cathodes and byabout 20 percent in comparison with that found in a similar bath withthe porous baffle but without the cooling of the cathodes.

In the following Table 1 the results of a series of tests are reported;these tests were carried out in a similar manner to the precedingExample and using different thermal conditions. For said tests a porousbaffle having pore-diameters of 35-45 1. was used. The consumption ofmaleic acid as well as the formation of precipitate generatingsubstances reported in the table are the values obtained after bringingthe bath up to its operating conditions.

TABLE 1 Formation of Temperature Temperature precipitate of the anodicof the cathodic Consumption generating Test compartment compartment ofmaleic substances No. C. C.) acid (gJAh) (gJAh) 22 22 1. 5-2 1 ca. 0.822 20. 5 ca. 1 1 ca. 0.1 28 20. 5 ca. 1 1 ca. 0.1 35 20. 5 ca. 1 1 ca.0.1 30 18.5 ca. 1 1 ca. 0.1 22 17 ca. 1 1 ca. 0.1 20 17 ca. 1 1 ca. 0.122 14 ca. 1 1 ca. 0.1 22 4 ca. 1 1 ca. 0. 1 22 -4 ca. 1 1 ca. 0.1 3522 1. 52 1 ca. 0.8 25 23 1.5-2 1 ca. 0.8 35 23 1. 5-2 1 ca. 0.8

1 Only in the cathodic compartment.

It will be understood that it is intended to cover all changes andmodifications of the example of the invention herein chosen for thepurpose of illustration which does not constitute departures from thespirit and scope of the invention.

What is claimed is:

1. In the process for self-coloring anodic oxidation of aluminum and itsalloys by means of an electrolytic bath consisting of an aqueoussolution of maleic and sulfuric acid, the improvement comprising placinga porous baffle immersed in the electrolytic solution used into theelectrolytic cell, said baffle having pores of a diameter between 0.1and 50 p. and subdividing the cell into an anodic and a cathodiccompartment;

maintaining the electrolytic solution in the cathodic compartment at atemperature below 22 C. and lower than that of the anodic compartmentbut higher than the temperature causing the freezing of the electrolyticsolution used in the cell; and applying direct current between theworkpiece as the anode and the cathode.

